Mystery Tern - Mick Cunningham's views

The following has been received from Mick, one of our most respected local birders, very valuable contribution here (despite his opening comment!)



WARNING – the following could be complete tosh! Photos are too hard.

Finally, have had a proper look. The only definite ID proposed on blog is Arctic. But apart from leg-length noted by DT, I can’t see anyone’s actually firmly asserted any feature as def diagnostic of Arctic (tho NCD I suspect knows in his own mind).

Age

I too think it has to be aged to start with. I don’t see anything on the bird that’s definite 1st summer – all the non-adult features I see are supportive of 1st W at least as much and often more so.

Scallops on upper mantle- Malling-Olson says diff between 1st W and 1st S is uppers just about plain grey with no marks or only faint ones on latter age (as Martyn saw). When pic is enlarged they look q obvious to me and do I detect a hint of brown in the scallops? If yes these must be retained recent juv. Those on the terts seem pretty strong to me too.

Primaries– I think there was mention of these poss being bleached/worn. They look new to me, hence the unworn, clean white rims. Again this = 1st W, whereas 1st summers would be worn as they’re old and their next moult won’t be until on winter quarters. (I know in adults fresh flight feathers have a pale bloom on them which lessens with wear, exposing dark base so feather becomes darker – which is why the new feathers on inner hand of common tern in spring are paler making the diagnostic dark upperwing wedge less obvious when they first return but darker as season goes on. I don’t know if same goes for juv flights, tho I read these get dark with wear so I assume so. The pic from Cambridge seems to show this. In any event, they’d look worn in some way. These don’t to me)

Primary Pattern – contra my earlier post, I think this can be used even on resting birds. And I think this is a non-subjective feature that shows it’s an Arctic. Initially, I thought it showed a solid white line right along the top edge of the folded wing. If this were so, it would have to be a Roseate. Indeed this feature is annotated in Collins on the plate on the juv Roseate. As the primary pattern is same at all ages this is important. Closer looks show the apparent white top border is illusion caused by diff positions of individual primaries/photo blur . If not, it’s a Roseate. Incidentally, Roseate has white tips to all primaries, even those with dark shafts, like Little gull rim – but it needs good pics to see this.

Outer Tail – Mr Dawtry is too tentative. I think the preening bird shows dark sides to outer tail. This rules out Roseate. Unless it’s photo effect.

Legs – when enlarged, the photos really do seem to show the legs are short – legless. Roseates are longer legged even than Common.

Head Pattern- I’m not sure I’ve fully understand what the lit says about the black lower edge on juv/1st W cap being a diff shape on Roseate but here goes: it seems to indicate the ear cov dark is angles at a sharp point. If I’ve got this right I think the (slightly?) more rounded lobe projecting down behind the eye in the photo = Arctic. I also think this rules out 1st S Arctic as the (limited) pics I’ve seen seem to show the lower edge of the cap has no lobe projecting down but cuts across mimicking shape of adult summer. Also, most 1st S pics I’ve seen show peppering in white forehead (dunno if this always holds).

Odd Stuff

Bill looks long to me – but I now read there’s overlap with the supposedly longer-billed Roseate.

Jizz looks diff in diff pictures – tho wings look long compared to length of whole bird as contrasted to roseate – but this is not scientific.

Call – did it sound like a sandwich ie a roseate??

Folk who saw it said they saw NO black rim on under wing tip. Worry is - why not??? Taking it other way, on the poor flights, the inner under-wing has an obvious pale window on inner prims and some kind of dark at tip – whether this is arctic rim or roseate dark and white lines up the individual outer primaries (venetian blind) is hard to say.

Nick D said coastal birders would laugh at our puzzlement. Maybe, but they usually have flight action to help and, even then, I’ve heard debates and fall outs. See what a mess some of em make of perched raptors and you won’t feel so bad.

Anyway, that’s my two penn-orth.

Interested to hear views – as Mr Garner says – “always learning”.